Microwave power divider



June 18, 1968 NER ET AL 3,389,350

MI CROWAVE POWER DIVIDER Filed Feb. 24, 1966 FIG. 2. PM

COUPLING (d b) 25 Use lfzs' L063 LOOP LENGTH (INCHES) FIG 4 H6 5' FREQUENCY (MC) United States Patent 3,389,350 MICROWAVE POWER DIVIDER George E. Sanner, Sparks, Md., and Otto Schmid, In, San

Jose, Calif., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 24, 1966, Ser. No. 529,761 5 Claims. (Cl. 333-9) This invention relates to microwave guide apparatus and is particularly directed to an improved power divider for microwave guides.

Objects of the invention are to produce novel and improved apparatus for coupling energy out of a microwave guide without producing mismatch of impedance which would produce undesirable reflections in the waveguide.

There exists a great need for microwave power dividing circuits which are simple, compact in arrangement, light in weight, and inexpensive and simple to manufacture. Such devices are particularly needed in the low power dividing systems, such as are used in stable local oscillator circuits for radar and similar equipment. Heretofore, the state of the art required that the power dividing circuits for this type of application be limited to the more cumbersome waveguide type. Magic Ts are frequently used but have obvious disadvantages, as well as being large and heavy. Coaxial Ts have also been used but are expensive. Microwave strip line techniques provide compactness and light weight but have the disadvantage of high attenuation due to the strip dielectric absorption.

This invention utilizes magnetic field coupling loops of novel and unusual shape. For years, the art has employed coupling loops for the purpose of coupling energy from one microwave circuit to another. However, such devices are said to be associated with impedance matching elements and each installation had to be carefully adjusted for maximum coupling with minimum losses. An example of this is the conventional technique of coupling energy from an echo box in a radar system. In such installations a circular or rectangular loop is used. However, as those skilled in the art are aware, it has never been practical to use this magnetic field coupling loop in a waveguide section for the purpose of coupling out a sizable percentage of the energy being propagated within the guide. The reason for this is simply that the terminal impedance of the loop differs so much from guide space impedance that an extremely great mismatch occurs. This is true for any sizable coupling case. Accordingly, the loop couplers of the prior art have been applicable only to very small coupling values, usually those greater than 30 db. Accordingly, it is a primary object of this invention to overcome the disadvantages of these prior art devices.

More specifically, another object is to provide a microwave coupling device, having wide band capabilities and providing very good impedance match between the guide and the coupling without any auxiliary impedance matching devices being required.

Another object is to provide a novel and improved microwave coupling device having such characteristics that many of the coupling devices can be coupled at closely spaced points on the waveguide without causing interactions to the coupling device and which will also have low insertion losses.

A still further object is to provide a microwave coupling device which is very small, compact, light in weight and inexpensive and simple to manufacture.

Other and further objects will become apparent from the following description when considered in connection with the accompanying drawings, in which:

FIGURE 1 is an isometric view, partly broken away, of a microwave guide embodying the present invention;

FIG. 2 is a cross sectional view on line 11-11 of FIG. 1, looking in the direction of the arrows;

3,389,350 Patented June 18, I968 FIG. 3 is a profile view of the waveguide centered on the section line of FIG. 2;

FIG. 4 is a graphical representation of the variation in coupling as a function of the length of the triangular coupling loop; and

FIG. 5 is a graphical representation of the VSWR versus frequency for microwave apparatus in accordance with the present invention.

Broadly speaking the present invention resides in the utilization of a triangular coupling loop with the necessary simple impedance matching means for providing a microwave power divider for coupling controllable amounts of power from a waveguide. It is well known that microwave energy can be coupled from a. waveguide using either means coupling with the H fields that is the magnetic field or the E field that is the electrostatic lines of force. It is also of course common knowledge that in a rectangular waveguide operating in the TE mode such as the guide illustrated in the drawings, that the E field is a maximum in the center and continues to diminish toward the narrow sides where it is Zero. Then assuming a point in the waveguide where the E fields are polarized with the arrows upwardly in the middle of the waveguide there will be an H field at right angles thereto circling the E field vectors. In other words, the H field is a maximum along the narrow walls of the guide and they continue to diminish toward zero in the center where the E field is a maximum. In prior devices which utilize rectangular loops or circular loops the plane of the loop has been so arranged and is of such size that it couples both with the E field and the H field, but it is apparent that since the two fields are at right angles to each other there must necessarily be substantial mismatch of the impedance. If the loop is oriented primarily to couple to the H field there will be inherently a substantial amount of the energy coupled from the E fields and this energy will be out of phase with the energy in the H field. In the present invention, utilizing the triangular coup-ling loop which has its apex pointing toward the center of the guide it will be apparent that the coupling loop is almost wholly within the H field gradient as shown in the drawings. In this manne the microwave energy generated in the loop results primarily from the H field linkage. Although it is well known in the art that a certain amount of E field linkage will also be present this latter amount is very small compared to that in a conventional rectangular loop. As will be described hereinafter it has been found that the amount of the coupling is primarily determined by the length of the loop, which is herein defined as the distance between the base and the apex of the loop which extends toward but does not extend entirely to the center of the rectangular microwave guide. In accordance with this invention it has been determined that this dimension can be increased so as to have a value which is a very high percentage of onehalf of the waveguide width and that excellent coupling adjustment can be achieved by adjusting the plane of the loop about an axis extending through the narrow sides of the rectangular waveguide.

As is well known the primary factor which determines the feasibility and practicability of devices for coupling microwave energy out of a waveguide is the extent to which the coupling device can be matched to the impedance in the waveguide in order that reflections will not develop. In accordance with the present invention it has been discovered that this proper impedance match can be accomplished by a very simple matching iris placed in close proximity to the coupling loop. By the proper adjustment and the position of the coupling iris and its relation to the triangular coupling loop the impedance mismatch can be minimized over a rather wide frequency band. By proper adjustment this mismatch can be minimized and the VSWR value can be maintained at values less than 1.2:1 or a bandwidth of the order of 15%.

Referring to FIG. 1 the present invention is illustrated in connection with a section of waveguide 10. The section of waveguide 10 is shown as being long enough to accommodate several of the coupling devices in order to illustrate that it is possible to extract microwave energy from fairly closely spaced points in a microwave guide without at the same time producing disturbing reflections which would make the conventional coupling devices impractical. Triangularly shaped coupling loops 11 are illustrated as being supported from one of the narrow side walls of the guide in such a manner that they can be rotated about an axis perpendicular to the side wall of the guide and passing through the apex 10a of the coupling loops. There is shown an arrangement, best illustrated in FIG. 5, wherein metal plugs 13 to which the triangular loops 10 are secured, are mounted for rotatable adjustment about axis perpendicular to the sides of the waveguides and bisection the apices of the coupling loops.

To this end, reinforcing plates 14 are soldered to the sidewall of the waveguide. The metal plug 13 is rotatable in an aperture in the sidewall and plate 14. A flange 16 fixed to the plug 13 is provided with arcuate slots 17. Suitable cap screws 18 extend through the slots and threadably engage the plate 14 for the purpose of holding the coupling loops in selected angular positions. The metal plug is a part of a coaxial cable terminal and are provided with suitable external threads 21 to take the female end of a coaxial cable.

As shown in FIG. 1 several of the triangular loops may be used for extracting energy from the microwave guide and may be spaced a minimum distance from each other of approximately one-half of an operating Wavelength in the microwave guide. Suitable simple impedance matching means are provided for each of the triangular loops in the form of a rectangular iris 22 which is mounted on the side of the center line opposite the triangular loop and 4. is spaced therefrom a selected distance Y which determines the bendwidth of the device. The distance Y and the dimension X are selected empirically in accordance with principles well known in the art for minimizing impedance mismatch.

We claim as our invention:

1. Waveguide apparatus com-prising a section of waveguide for propagating microwaves, means for extracting microwave energy from said guide comprising a triangular loop having its plane orthogonal to the longitudinal axis of said guide and having its apex adjacent the center of said waveguide.

2. Waveguide apparatus as described in claim 1 wherein said waveguide is a rectangular guide operating in the dominant TE mode, the base of said loop being adjacent one of the short sides of said guide.

3. Waveguide apparatus as set forth in claim 1 wherein the base of said triangular loop is substantially equal in length to a narrow sidewall of said guide.

4. Waveguide apparatus as set forth in claim 1 plus impedance matching means comprising a rectangular iris plate located in the waveguide wholly on the opposite side of the center line of the waveguide from said triangular loops and positioned to be cut by the H field which links the triangular loop.

5. Waveguide apparatus as set forth in claim 1 and including means for mounting said triangular coupling loops for adjustable rotation about axes perpendicular to the sidewalls of said guide and bisecting the respective angles at the apices of said loops.

References Cited UNITED STATES PATENTS 3,176,300 3/1965 Kuecken 333--21XR HERMAN KARL SAALBACH, Primary Examiner.

M. NUSSBAUM, Assistant Examiner. 

1. WAVEGUIDE APPARATUS COMPRISING A SECTION OF WAVEGUIDE FOR PROPAGATING MICROWAVES, MEANS FOR EXTRACTING MICROWAVE ENERGY FROM SAID GUIDE COMPRISING A TRIANGULAR LOOP HAVING ITS PLANE ORTHOGONAL TO THE LONGITUDINAL AXIS OF SAID GUIDE AND HAVING ITS APEX ADJACENT THE CENTER OF SAID WAVEGUIDE. 